Galilean satellite images from Galileo, considered in the
context of Shoemaker-Levy 9 impacts and other evidence, raise
crucial questions about the nature of impactor populations in the
Jovian system. Several observations suggest that the current impactor
population (generally believed to be extinct Jupiter-family comets) is
relatively deficient in comets smaller than S-L 9, compared with
usually adopted power-laws. These include: (a) lack of saturated
surfaces at diameters <100 m; (b) predominance of secondary craters in
some regions far from sources; and (c) apparent dearth of small crater
chains (catenae). None of these indications is yet conclusive (for
example, catenae would be predicted to mimic the cometary size
distribution only for certain idealized models of S-L 9-like comet
break-up). It is also plausible (certain, in the case of Europa) that
there are currently active erosional and resurfacing processes on
Europa, Ganymede, and Callisto that contribute to erasing small craters.
The often adopted assumption that the heavily cratered terrains on
Ganymede and Callisto reflect the same Late Heavy Bombardment recorded
on the Moon, while conceivably true, is only one of a number of
plausible possibilities. We discuss the qualitative roles of different
impactor populations.

We thank the Galileo Imaging Team and associates for discussions,
and the Galileo Project, NASA, and NSF for support.